Genetic and environmental dissection of short and long-term social aggression in pigs
It is common for pigs to engage in physical aggression when mixed into new social groups, in order to establish dominance relationships. Phenotyping aggression is time consuming, however skin lesions resulting from physical aggression are quick to record, are genetically correlated with aggressive behavioural traits, and have low to moderate heritability (0.19 to 0.43). Reducing aggression via selection on skin lesion traits would provide a socially acceptable, long-term solution to the problem. Barriers to implementing selection against skin lesions lie in the lack of understanding regarding the underlying genetic basis of aggression, and its relationship with other behaviour and production traits. This thesis has focused on dissecting the phenotypic and genetic relationship between skin lesions recorded 24 hours after mixing (SL24h), and either 3 or 5 weeks later (SL3wk/SL5wk, respectively), with aggression performed at mixing, and several production traits. Chapter 2 provided evidence of a potential trade-off between involvement in aggression upon first mixing, and receipt of aggressive attacks several weeks after mixing. In particular, animals that avoid aggression at mixing had the highest fresh skin lesion numbers at 3 weeks. This suggests that reciprocal fighting at mixing may be beneficial for long-term group social stability. It also suggests that it may be possible to phenotype the least aggressive individuals in a group using SL3wk. In Chapter 3, I quantified the magnitude of reduction in complex aggressive behavioural traits when using SL24h or SL3wk as selection criteria, to identify the optimum skin lesion trait for selection purposes. The results of Chapter 3 provided evidence that selection against anterior SL24h would result in the greatest genetic and phenotypic reduction in aggressive behaviour recorded at mixing. Although there is evidence that selection for increased SL3wk would reduce aggression at mixing, current understanding of aggressive behaviour under stable group conditions is insufficient to recommend using this trait for selection purposes. Chapter 4, presented genetic associations between skin lesion traits as a measure of short- and long-term aggression, and commonly used commercial performance measures: growth, feed intake, feed efficiency, and carcass traits. The results suggested that, genetically, animals that receive many lesions show improved performance compared to those with few lesions, except for anterior SL24h, which have been shown to be genetically positively correlated with the initiation of nonreciprocal attacks. The aim of Chapter 5, was to determine whether skin lesion traits are phenotypically or genetically associated with behavioural measures of fearfulness. As found in Chapter 4, there was some evidence of an association between SL5wk and the traits, however this was not the case for anterior SL24h. For the 6th and final Chapter, we used skin lesion data from 1,840 pigs to perform genome wide association studies (GWAS), which detected a single SNP significantly associated with SL5wk on a genome wide level, as well as several SNPs associated with both SL24h and SL5wk on a chromosome wide level.